For The Fruit For The Topping

1. Heat the oven to 200°C/180°C fan/400°F/gas mark 6.
2. Slice the rhubarb and apple into bite-sized pieces. Add them to a pan along with the sugar, lemon juice, and vanilla extract.
3. Cook over low heat until the fruit softens. Then, transfer the fruit mixture to an approx 20 x 24 cm baking dish or a deep 23 cm/9 in pie dish.
4. Combine the flour, cinnamon, and cold butter in a bowl. Use your fingertips to rub the butter into the flour until the mixture resembles breadcrumbs.
5. Add the sugar, oats, and chopped hazelnuts to the flour mixture.
6. Crumble the topping over the fruit and gently press down.
7. Bake for approximately 25 minutes, or until golden brown and bubbly.